An electric power steering system, which applies the driving force of a motor directly to a steering line so as to supply assist steering torque, can assist steering made by a driver. In the electric power steering system, an electric control unit (ECU) determines target current to be supplied to the motor according to steering torque, vehicle velocity and the like, making a comparison between the target current and actual current flowing into the motor. And a deviation, the result of comparison, which first experiences processing executed by a proportional and integral controller, is then supplied for controlling the motor. A control method of this type is reported in Japanese Published Patent Application 7-196048.
The ECU, which has a microcomputer for controlling the electric power steering system, executes current feedback so that a target current is in agreement with an actual current, in other words a deviation between these currents becomes zero. In a feedback loop including the ECU and microcomputer, an attenuator is employed so as to increase or decrease the deviation according to vehicle velocity, namely to make the response of feedback loop variable according to the vehicle velocity. In Japanese Patent No. 3152339 an attenuator of this kind is reported. The gain of attenuator is adapted to be higher as a vehicle runs fast so as to achieve better response and to be lower as the vehicle runs slow to restrict generation of noise, so that performance in terms of better response can coexist with commercial value associated with noise reduction.
Applicants have discovered that steering feeling is sometimes degraded due to lack of appropriate assist by a motor when sudden steering is made during low speed running of a vehicle. This phenomenon is attributed to the fact that the attenuator has a gain which is adjusted to be compatible for both conditions of low and high speed running. Also the applicants have discovered that the oscillation of motor current sometimes occurs if sudden steering is made during very low speed maneuvering or stopping of a vehicle when larger reaction force is exerted by a road surface. Though the oscillation lasts for a short period of time when sudden steering is being made, it possibly causes mechanical vibration, the frequency of which sometimes happens to fall in an audible range of man, thereby creating irritating noise to make a driver uncomfortable. This could lead to damaging of commercial value. Even if the oscillation does not develop so far, it is not desirable to create noise and vibration.